1. Field of the Invention
The present invention relates generally to microcontrollers and, in particular, to a simple, cost-effective controller architecture for the reliable, serial mode programming of an on-chip EPROM utilizing minimal resources.
2. Discussion of the Prior Art
Microcontroller devices typically include an on-chip non-volatile memory element which stores application software. If this memory is ROM, then the application program is fabricated into the device during manufacturing and can never be changed. This one time programming problem is solved by making the ROM electrically programmable (EPROM).
A microcontroller with on-chip EPROM usually has a number of distinct operating modes, typically including at least one programming mode for loading application data into the EPROM and for reading the EPROM contents. The microcontroller operating modes, of course, include a normal run mode which is utilized when the EPROM has been programmed and the device is performing its intended function of executing its application code.
In the EPROM programming mode, the microcontroller is configured to emulate a stand-alone EPROM memory device. In this way, the programming of microcontrollers can be accomplished using well accepted and well understood methods and equipment for programming EPROMs. This is done by allowing the microcontroller EPROM to be programmed by an external programming system.
However, to program an EPROM, the programming hardware must have access to it. The details of this access and its execution depends upon how the microcontroller logic provides a programming interface between the external programming system and the internal on-chip EPROM. In the past, this interface has always used a parallel data-flow architecture, which is silicon intensive.